Astragalus megacarpus (Nutt.) A.Gray
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Authors
Rupert C. Barneby
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Authority
Barneby, Rupert C. 1964. Atlas of North American Astragalus. Mem. New York Bot. Gard. 13(2): 597-1188.
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Family
Fabaceae
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Scientific Name
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Type
"Plains of the Rocky Mountains, Nuttall."—Holotypus, labeled by Nuttall "Phaca *megacarpa. R. Mts., BM! isotypi, GH, NY, PH!
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Synonyms
Phaca megacarpa Nutt., Tragacantha megacarpa (Nutt.) Kuntze, Astragalus megacarpus var. parryi A.Gray, Astragalus megacarpus var. prodigus E.Sheld.
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Description
Species Description - Low, robust, subacaulescent or shortly caulescent, commonly appearing glabrous but the leaflets thinly ciliate and strigulose on the midrib beneath with straight, appressed hairs up to 0.25-0.5 (0.6) mm. long, more rarely cinereous, especially when young; stems numerous, 1—5 cm. long, arising from scaly caudex-branches at or just below soil-level, simple or spurred at 1—2 nodes below the first peduncle, together forming dense, leafy tufts 1—4 dm. in diameter, the internodes mostly concealed by imbricated stipules, if one or more developed then scarcely over 1 cm. long; stipules submembranous becoming papery, usually several-nerved, broadly ovate or ovate-acuminate, 3—7 mm. long, amplexicaul-decurrent around the whole stem’s circumference but free, forming a loose, pallid, purplish, or castaneous sheath, glabrous or strigulose dorsally, ciliate; leaves erect, (2) 5-17 cm. long, with stiff, subpersistent petioles and (7) 9-15 broadly obovate, ovate, oblong-elliptic, or suborbicular, obtuse or shallowly retuse, commonly callous- mucronulate, flat leaflets 5—14 mm. long; peduncles 0.5-2.5 (7) cm. long, much shorter than the leaves; racemes loosely 3-5 (8)-flowered, the flowers ascending, the axis usually less than 1 (rarely up to 2.5) cm. long in fruit; bracts papery- membranous, ovate or ovate-acuminate, 2—5 mm. long; pedicels ascending, straight or nearly so, at anthesis 3.5-5 mm., in fruit 5-8 mm. long; bracteoles 0-2, attached to the pedicel when present; calyx 8.5-13.5 (16) mm. long, strigulose with black or mixed black and white hairs, the strongly oblique disc 1—1.5 (2.5) mm. deep, the cylindric tube 6-10 mm. long, 3—4.3 (5) mm. in diameter, the subulate or lance-subulate teeth 1.9-4.5 (6) mm. long, the ventral sinus cut back and the orifice oblique; petals pink-purple, or white with pale pink veins; banner gently recurved through ± 45°, oblanceolate or rhombic-oblanceolate, 16.5-22 (26) mm. long, 7.5-11 (12.4) mm. wide; wings 14.5-20 (21.5) mm. long, the claws 7-11 mm., the lanceolate, straight or slightly incurved blades 9-11.5 (12.2) mm. long, 2-3 mm. wide below the middle, commonly tapering upward into a narrow but obtuse apex, more rarely oblong and emarginate; keel 11.5-17.5 mm. long, the claws 6.5-11 mm., the half-obovate or lunately half-elliptic blades 5.5-7.5 mm. long, 2.5-3.6 mm. wide, incurved through 75-85 (90)° to the rounded or sharply deltoid apex; anthers 0.7-0.9 mm. long; pods ascending (mostly humistrate), appearing radical, gathered into a ring on the ground beneath the tufted foliage, the gynophore 2-4 mm. long, the body symmetrically or somewhat obliquely ellipsoid or ovoid-ellipsoid, bladdery-inflated, (3.5) 4-6 cm. long, 1.5-2 (or when pressed apparently to 2.5) cm. in diameter, broadly obconic at base, contracted distally into a triangular, laterally compressed, often incurved beak, otherwise subterete or shallowly sulcate ventrally, the papery, thinly strigulose valves at first greenish but commonly either speckled or brightly mottled with reddish-purple, becoming brownish-stramineous and somewhat lustrous, finely reticulate, not inflexed, the funicular flange 0.8—1.4 mm. wide; ovules 38-54; seeds mahogany-brown turning soot-black, minutely punctate, dull, 2.7-3.4 mm. long.
Distribution and Ecology - Gullied hillsides, knolls, and canyon benches, in stiff clays or gravels derived from limestone, shale, or red sandstone, 4900-7600 (10,000) feet, locally plentiful but scattered, the known stations concentrated into four more or less disjunct areas: relatively common in central and southwestern Utah, from the Wasatch Plateau in Emery County south to the headwaters of the Virgin in Kane and Iron Counties; local in the Uinta and Green River Basins, Duchesne County, Utah, Rio Blanco County, Colorado, and Uinta and Sweetwater Counties, Wyoming; apparently isolated on the upper North Platte in Converse County, Wyoming; and on calcareous gravel knolls and hilltops in Eureka and southern Nye Counties, Nevada.—Map No. 104.—Late April to July.
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Discussion
Jones (1923, p. 120) has characterized A. megacarpus as "very variable," and even if this opinion was colored by misidentified collections of A. oophorus var. caulescens from Gunnison, Colorado, and from northern Arizona, there is some truth in his statement. The leaflets vary in outline, the flowers vary in size and coloring, and the leaves may be either cinereous or almost hairless. Variation in density of pubescence is partly a seasonal phenomenon, for foliage ashen in early spring may turn green and leathery as summer advances. In the Zion region a variant with leaflets of an elliptic rather than the commoner obovate type is either prevalent or predominant; and this character, combined with supposedly narrower pods, provided the chief differential character of the original var. Parryi. However Jones maintained var. Parryi (1923, p. 120) principally on account of its purple flowers and assigned it a range extending (if one ignores misplaced elements of var. caulescens) from Duchesne County (Theodore) south along the Wasatch to the head of the Sevier and Virgin Rivers. The flowering season of A. megacarpus is early and short; consequently the majority of herbarium specimens are in fruit, leaving the question of petal-color open in many instances. So far as I can learn, the flowers are purple in the range indicated by Jones and in the disjunct stations in Nevada; they are white or white with a faint flush of pink or a flare of pink veins in the banner throughout Wyoming and along the White River in northwestern Colorado. A racial division on these lines is possible, but doubts about flower-color in many populations need to be dispelled before the existence of discrete entities can be established.
Whether white or purple, the flowers of the great bladdery milk-vetch appear in late April and May, expanding in clusters near the ground before the leaves are fully developed. The leaves elongate rapidly during and after anthesis and the short subradical peduncles bend out to carry the enormous inflated fruits in a crowded ring, sheltering under and surrounding the central more or less erect, leafy tuft.
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Objects
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Distribution
Utah United States of America North America| Colorado United States of America North America| Wyoming United States of America North America| Nevada United States of America North America|